Transistors having gates with a lift-up region

ABSTRACT

In accordance with at least one embodiment of the invention, a transistor comprises a semiconductor, a first drift layer, a drain region, a body region, a source region, a shallow trench isolation region, a dielectric, and a gate. The first drift layer is formed in the semiconductor and has majority carriers of a first type. The drain region is formed in the first drift layer and has majority carriers of the first type. The body region is formed in the semiconductor and has majority carriers of a second type. The source region is formed in the body region and has majority carriers of the first type. The shallow trench isolation region is formed in the first drift layer and disposed between the drain region and the body region. The dielectric is formed on the semiconductor, and the gate is formed over the dielectric and has a lift-up region.

BACKGROUND

LDMOSs (Laterally Diffused MOSFET) are transistors that find wide use inmany high-voltage switching applications, for example in switchingDC-to-DC converters. To reduce the size of inductors used in someDC-to-DC converters, an LDMOS is switched on and off at a relativelyhigh frequency.

SUMMARY

In accordance with at least one embodiment of the invention, atransistor comprises a semiconductor, a first drift layer, a drainregion, a body region, a source region, a shallow trench isolationregion, a dielectric, and a gate. The first drift layer is formed in thesemiconductor and has majority carriers of a first type. The drainregion is formed in the first drift layer and has majority carriers ofthe first type. The body region is formed in the semiconductor and hasmajority carriers of a second type. The source region is formed in thebody region and has majority carriers of the first type. The shallowtrench isolation region is formed in the first drift layer and disposedbetween the drain region and the body region. The dielectric is formedon the semiconductor, and the gate is formed over the dielectric and hasa lift-up region.

In accordance with at least one embodiment of the invention, thetransistor further comprises a doped region formed in the first driftlayer, where the doped region has majority carriers of the first type.

In accordance with at least one embodiment of the invention, thedielectric has a lift-up region under the lift-up region of the gate.

In accordance with at least one embodiment of the invention, the dopedregion shares a rounded interface with the shallow trench isolationregion.

In accordance with at least one embodiment of the invention, the roundedinterface reduces a local electric field during operation of thetransistor.

In accordance with at least one embodiment of the invention, thetransistor further comprises a second drift layer formed in thesemiconductor, where the second drift layer has majority carriers of thefirst type.

In accordance with at least one embodiment of the invention, a methodcomprises forming in a semiconductor a first drift layer having majoritycarriers of a first type, forming a shallow trench isolation region inthe first drift layer, growing a pad oxide layer over the semiconductor,depositing a nitride layer over the pad oxide layer, depositing aphotoresist layer over the nitride layer, exposing an opening pattern inthe photoresist layer, etching an opening in the photoresist layer basedon the opening pattern to expose an opening to the nitride layer,etching the opening to the nitride layer to expose an opening to the padoxide layer, removing the photoresist layer, growing oxide on theopening to the pad oxide layer, removing the nitride layer, removing thepad oxide layer, and leaving at least part of the oxide grown on the padoxide layer, growing a gate oxide layer, forming a gate over the gateoxide layer, forming in the first drift layer a drain region havingmajority carriers of the first type, forming in the semiconductor a bodyregion having majority carriers of a second type, and forming in thebody region a source region having majority carriers of the first type.

In accordance with at least one embodiment of the invention, where whengrowing the oxide on the opening to the pad oxide layer, the oxide isgrown to a thickness of at least 200 angstroms.

In accordance with at least one embodiment of the invention, the methodfurther comprises implanting dopants through the opening to the padoxide layer to provide majority carriers of the first type.

In accordance with at least one embodiment of the invention, for themethod, the majority carriers of the first type are electrons and themajority carriers of the second type are holes, wherein implantingdopants through the opening to the pad oxide layer includes implantingphosphorous or arsenic at a dose of 6·10¹¹ cm⁻² to 9·10⁻¹² cm⁻² withenergy in the range of 25 KeV to 250 KeV, with implant angles from 0° to9°.

In accordance with at least one embodiment of the invention, for themethod, the semiconductor comprises silicon. The oxide, pad oxide layer,and gate oxide layer each comprises silicon dioxide. Furthermore,growing the oxide on the opening to the oxide pad layer includesoxidizing the semiconductor.

In accordance with at least one embodiment of the invention, the methodfurther comprises forming in the semiconductor a second drift layerhaving majority carriers of the first type.

In accordance with at least one embodiment of the invention, forming inthe semiconductor the first drift layer comprises implanting arsenic inthe semiconductor with a dose of 8·10¹¹ cm ⁻² to 2·10¹³ cm⁻² at anenergy of 25 KeV to 400 KeV, with implant angles from 0° to 9°.Furthermore, forming in the semiconductor the second drift layercomprises implanting phosphorus in the semiconductor with a dose of1·10¹² cm⁻² to 2·10¹³ cm⁻² at an energy of 160 KeV to 1 MeV, withimplant angles from 0° to 9°.

In accordance with at least one embodiment of the invention, forming inthe semiconductor the first buried layer comprises implanting boron intothe semiconductor with a dose of 1·10¹² cm⁻² to 2·10¹³ cm⁻² at an energyof 800 KeV to 2 MeV, with implant angles from 0° to 9°.

In accordance with at least one embodiment of the invention, the methodfurther comprises forming in the semiconductor a second buried layerhaving majority carriers of the second type.

In accordance with at least one embodiment of the invention, a secondtransistor comprises a semiconductor, a first drift layer, a drainregion, a body region, a source region, a shallow trench isolationregion, a dielectric, and a doped region. The first drift is formedlayer in the semiconductor and has majority carriers of a first type.The drain region is formed in the first drift layer and has majoritycarriers of the first type. The body region is formed in thesemiconductor and has majority carriers of a second type. The sourceregion is formed in the body region and has majority carriers of thefirst type. The shallow trench isolation region is formed in the firstdrift layer and is disposed between the drain region and the bodyregion. The dielectric is formed on the semiconductor, and the dopedregion is formed in the first drift layer and has majority carriers ofthe first type.

In accordance with at least one embodiment of the invention, for thesecond transistor above, the doped region shares a rounded interfacewith the shallow trench isolation region.

In accordance with at least one embodiment of the invention, for thesecond transistor above, the rounded interface reduces a local electricfield during operation of the transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 shows an illustrative LDMOS in accordance with various examples;

FIG. 2 shows fabrication of an illustrative LDMOS in accordance withvarious examples;

FIG. 3 shows fabrication of an illustrative LDMOS in accordance withvarious examples;

FIG. 4 shows fabrication of an illustrative LDMOS in accordance withvarious examples;

FIG. 5 shows fabrication of an illustrative LDMOS in accordance withvarious examples;

FIG. 6 shows fabrication of an illustrative LDMOS in accordance withvarious examples;

FIG. 7 shows fabrication of an illustrative LDMOS in accordance withvarious examples; and

FIG. 8 shows illustrative process steps to fabricate an illustrativeLDMOS in accordance with embodiments.

DETAILED DESCRIPTION

LDMOSs are switched on and off at a relatively high frequency to reducethe size of inductors used in some DC-to-DC converters. However, highfrequency switching may lead to energy losses due to the gate-to-sourcecapacitance and gate-to-drain capacitance of an LDMOS. Furthermore, anLDMOS may exhibit hot carrier degradation, thereby adversely affectingthe LDMOS characteristics and manufacturability. It is desirable toprovide LDMOSs suitable for high frequency switching, with an acceptablefigure-of-merit R_(SP)Q product, where R_(SP) is the specificdrain-to-source on-resistance and Q is the gate charge.

In accordance with the disclosed embodiments, a transistor, such as anLDMOS, comprises a gate having a lift-up region. The lift-up region isproposed to help reduce the capacitance between the gate and the driftlayer of the LDMOS. An LDMOS may comprise a shallow trench isolation(STI) region having a rounded corner in the junction field effecttransistor (JFET) region of the LDMOS. The rounded corner is proposed tohelp improve the interface between the STI region and the JFET region,and to help provide reduced surface field (RESURF), as well asmitigation of hot carrier degradation. In some embodiments, doping ofthe JFET region may be fine-tuned by self-aligned implanting in the JFETregion. In accordance with the disclosed embodiments, an LDMOS comprisesa deep p-type buried layer adjacent to a p-type buried layer, and forsome embodiments comprises two n-type drift layers, where the p-type andn-type layers have graded doping profiles for RESURF and for providingfully depleted regions under high voltage operation.

FIG. 1 shows an illustrative LDMOS 100 according to an embodiment. Theillustrative LDMOS 100 is formed in a p-type silicon substrate 102 thatmay initially be lightly doped p− to highly doped p+. For ease ofillustration, FIG. 1 does not show all typical elements of an LDMOS,such as various metal and dielectric layers formed above the activeregion in the silicon substrate 102 as part of the BEOL (Back End OfLine) fabrication of an integrated circuit device. FIG. 1 illustrates asimplified cross-section of the illustrative LDMOS 100, where theillustrated features are not drawn to scale.

Formed in the silicon substrate 102 are an n-type buried layer 104, ap-type buried layer 106, and a p-type buried layer 108. In someembodiments, the p-type buried layer 106 is formed by implanting boroninto the silicon substrate 102 with a dose of 3·10¹² cm⁻² to 8·10¹² cm⁻²at an energy of 800 KeV to 2.5 MeV. In some embodiments, the p-typeburied layer 108 is formed by implanting boron into the siliconsubstrate 102 with a dose of 1 10¹² cm⁻² to 2·10¹³ cm⁻² at an energy of800 KeV to 2 MeV, with implant angles from 0° to 9°.

The relationship among the layers in the illustrative LDMOS 100 may bedescribed as the p-type buried layer 108 being formed on the p-typeburied layer 106, and the p-type buried layer 106 being formed on then-type buried layer 104, where the p-type buried layer 108 and thep-type buried layer 106 are adjacent to each other, and the p-typeburied layer 106 and the n-type buried layer 104 are adjacent to eachother. However, it is to be appreciated that these layers may not haveprecisely defined boundaries where one layer stops and an adjacent layerbegins.

Formed in the silicon substrate 102 are an n-type drift layer 110 and ann-type drift layer 112. In some embodiments, the n-type drift layer 110is formed by implanting phosphorus with a dose of 1·10¹² cm⁻² to 2·10¹³cm⁻² at an energy of 160 KeV to 1 MeV, with implant angles from 0° to9°. In some embodiments, the n-type drift layer 112 is formed byimplanting arsenic with a dose of 8·10¹¹ cm⁻² to 2·10¹³ cm⁻² at anenergy of 25 KeV to 400 KeV, with implant angles from 0° to 9°. Then-type drift layer 110 and the n-type drift layer 112 may be describedas being adjacent to each other, where the n-type drift layer 112 isformed on the n-type drift layer 110. The n-type drift layer 110 may bedescribed as being adjacent to and formed on the p-type buried layer108. However, these layers may not have precisely defined boundarieswhere one layer stops and an adjacent layer begins.

In some embodiments, the order in forming the layers 104, 106, 108, 110,and 112 is implied by their ordered illustration in FIG. 1, where then-type buried layer 104 is formed in the silicon substrate 102 beforethe p-type buried layer 106, and the layer p-type buried layer 108 isformed after forming the p-type buried layer 106, followed by formingthe n-type drift layer 110 and then the n-type drift layer 112. For someembodiments, the same mask may be utilized when forming the layers 104,106, 108, 110, and 112.

An STI region 114 is formed in the n-type drift layer 110 and in then-type drift layer 112. A drain region 116 is formed in the n-type driftlayer 112 adjacent to the STI region 114. For the embodiment of FIG. 1,the drain region 116 is highly doped n-type. A contact 118 is made tothe drain region 116 to provide electrical connection from the drainregion 116 to other vias and metal layers, not shown. The STI region 114is a dielectric and may comprise SiO₂ (silicon dioxide).

A gate 120 is formed over a gate oxide region 119, above part of STIregion 114, and includes a lift-up region 122. The gate 120 may comprisepolysilicon. The lift-up region 122 arises because the STI region 114has a lift-up region 124, and the lift-up region 124 is due to edgeoxide growth of the STI region 114. This growth also causes a roundedcorner 126, as will be described in more detail later. The lift-upregions 122 and 124 are proposed to help reduce the gate-to-draincapacitance of the illustrative LDMOS 100, and the rounded corner 126 isproposed to provide RESURF.

The lift-up regions 122 and 124 may be described as being proximal toeach other. In the embodiment of FIG. 1, this proximity is due to thegate 120 conforming to the surface of the STI region 114 duringfabrication as the gate 120 is formed adjacent to the STI region 114.

A p-type body region 128 is formed, adjacent to the n-type drift layers110 and 112, where for some embodiments the gate 120 may provideself-alignment of the p-type body region 128. For some embodiments, thep-type body region 128 may be formed before forming the gate 120. Thep-type body region 128 may be described as being adjacent to the n-typedrift layers 110 and 112, so that the STI region 114 is disposed betweenthe drain region 116 and the p-type body region 128. A non-heavily dopedn-type region 130 may be formed in the p-type body region 128, followedby depositing a spacer layer 132 adjacent to the gate 120, followed byforming a heavily doped n-type region 134 so that the regions 130 and134 provide a source region. A body contact 136 is a heavily dopedp-type region in the p-type body region 128 to provide an ohmic contactto the p-type body region 128.

In some embodiments, to form part of the source region (the region 130),an upper region of the p-type body 128 may be implanted with arsenicwith a dose of 3·10¹³ cm⁻² to 1·10¹⁵ cm⁻² at an energy of 25 KeV to 160KeV, with implant angles from 0° to 9°. In some embodiments, the upperregion of the p-type body 128 may also be implanted with boron with adose of 1·10¹³ cm⁻² to 5·10¹⁴ cm⁻² at an energy of 60 KeV to 260 KeV,with implant angles from 7° to 35°, for the body contact 136. In someembodiments, a bottom region of the p-type body 128 may be implantedwith boron with a dose of 2·10¹² cm⁻² to 6·10¹³ cm⁻² at an energy of 300KeV to 1.6 MeV, with implant angles from 0° to 9° to connect the p-typebody 128 to the p-type buried layer 108 for RESURF.

A contact 138 is formed on the source region (region 134) and the bodycontact 136. Vias and other metal layers (not shown) are electricallyconnected to the contact 138 to provide electrical connection to thep-type body 128 and to the source region (regions 130 and 134).

Portions of the p-type body 128 and the n-type drift layer 112 that arenear each other and share an interface define a JFET region. In someembodiments, before the gate oxide region 119 and the gate 120 areformed, a region 139 in the JFET region, specifically in the n-typedrift layer 112 adjacent to the STI region 114, is doped with implantsto fine-tune the electric field in the JFET region. The region 139shares a rounded interface with the rounded corner 126 of the STI region114. The region 139 is referred to as a JFET adjusting implant region139.

Some embodiments may have one p-type buried layer instead of the twop-type buried layers 106 and 108, and one n-type drift layer instead ofthe two n-type drift layers 110 and 112. However with the layers 106,108, 110, and 112 having a balanced doping profile, the illustrativeLDMOS 100 may be better (or fully) depleted under reverse bias so thathigher voltage circuits may be realized, with a relatively small driftlayer.

The illustrative LDMOS 100 is an n-channel LDMOS, where the majoritycarriers of the n-type regions are electrons and the majority carriersof the p-type regions are holes. Other embodiments may interchange then-type regions and p-type regions so that a p-channel LDMOS may befabricated.

An STI region 140 may be formed when the STI region 114 is formed, wherethe STI region 140 helps provide isolation from other devices (notshown). A deep trench comprising vertical layers of n-type, p-type, andoxide layers may be formed next to the illustrative LDMOS 100 to providefurther isolation, but for ease of illustration these layers are notshown.

FIG. 2 illustrates fabrication of the illustrative LDMOS 100 accordingto an embodiment, where the STI regions 114 and 140 are formed afterforming the layers 104, 106, 108, 110, and 112. The STI regions 114 and140 may be formed with the same process steps, where their respectivetrenches are etched and lined with an oxide liner (not shown), an oxide(SIO₂) is deposited over the oxide liner, and CMP (chemical mechanicalpolishing) is performed. (FIG. 1 shows the STI region 140 somewhatthicker than as shown in FIG. 2 because of additional oxide growth atlater process steps.) A sacrificial pad oxide layer 202 is grown overthe STI regions 114 and 140 and the n-type drift layer 112.

FIG. 3 illustrates fabrication of the illustrative LDMOS 100 accordingto an embodiment, where a nitride (Si₃N₄) layer 302 and a photoresistlayer 304 are formed over the sacrificial pad oxide layer 202. Anopening pattern 306 is exposed in the photoresist layer 304 and etchedto expose an opening to the nitride layer 302, followed by etching theopening to the nitride layer 302 to expose an opening to the sacrificialpad oxide layer 202.

FIG. 4 illustrates fabrication of the illustrative LDMOS 100 accordingto an embodiment, where arrows 402 indicate the step of implantingdopants into that part of the n-type drift layer 112 that will be partof the JFET region and that will form the JFET adjusting implant region139. The dopants are implanted through the opening to the sacrificialpad oxide layer 202. In the embodiment of FIG. 4, the dopants are n-typedonors.

FIG. 5 illustrates fabrication of the illustrative LDMOS 100 accordingto an embodiment, illustrating the JFET adjusting implant region 139resulting from the doping step of FIG. 4. For some embodiments, to formthe JFET adjusting implant region 139, phosphorous or arsenic may beimplanted at a dose of 6·10¹¹ cm⁻² to 9·10¹² cm⁻² with energy in therange of 25 KeV to 250 KeV, and with implant angles from 0° to 9° tocompensate for interface states and to reduce the JFET regionresistance.

Doping the JFET region of the illustrative LDMOS 100 to provide the JFETadjusting implant region 139 can reduce the JFET region resistance andallow for adjustment of the electric field in the JFET region. Chainimplantation to provide the doping profiles of the JFET region, then-type drift layers 110 and 112, and the p-type buried layers 106 and108 allows for trading off performance with the gate length (i.e., thegate length associated with the gate 120), hot carriers, and drift layerresistance. The doping profile can help shield the electric fieldbetween the p-type body region 128 to a sidewall of the STI region 114(e.g., the rounded corner 126 in FIG. 1) and the p-type buried layer 108to that part of the n-type drift layer 110 below the bottom of the STIregion 114. This can help mitigate surface punch-through of theresulting small channel, so that the illustrative LDMOS 100 may realizea relatively small gate length with low leakage and a reducedgate-to-source capacitance.

FIG. 6 illustrates fabrication of the illustrative LDMOS 100 accordingto an embodiment. The photoresist layer 304 is removed, followed bythermal oxide growth on the opening to the sacrificial pad oxide layer202, resulting in the lift-up region 124. In some embodiments, theadditional oxide grown on the opening to the sacrificial pad oxide layer202 may have a thickness greater than 200 angstroms, for example from360 angstroms to 1600 angstroms, and grown at a temperature between 760°C. to 990° C. The sacrificial pad oxide layer 202 when first formed isabout 100 angstroms thick, so that the additional oxide due to thermalgrowth is substantially larger than the initial thickness of thesacrificial pad oxide layer 202. The figures are not drawn to scale, andtherefore do not depict the scale of the oxide growth relative to thesacrificial pad oxide layer 202. There is also growth of the STI region114 into the n-type drift layer 112, resulting in the rounded corner126.

FIG. 7 illustrates fabrication of the illustrative LDMOS 100 accordingto an embodiment. After the thermal growth of oxide depicted in FIG. 6,the nitride layer 302 is stripped off, a photoresist layer (not shown)is deposited, patterned, and etched for the body region, and dopants areimplanted to form the body region 128. The photoresist layer is removed,and the sacrificial pad oxide layer 202 is removed. Because the oxidethickness due to thermal growth is much larger than the initialthickness of the sacrificial pad oxide layer 202, removal of thesacrificial pad oxide layer 202 still leaves a relatively substantialthickness of oxide in the lift-up region 124. A high quality gate oxideis thermally grown after removing the sacrificial pad oxide layer 202,resulting in the oxide layer 119 depicted in FIG. 7.

The gate 120 is formed on the oxide layer 119 over portions of the bodyregion 128, part of the n-type drift layer 112 and the JFET adjustingimplant region 139, and the STI region 114, as shown in FIG. 7. The gate120 may be formed by depositing polysilicon on the oxide layer 119,depositing a photoresist layer on the polysilicon, patterning andetching away exposed parts of the photoresist layer to expose unwantedportions of the polysilicon, followed by removing the unwantedpolysilicon. A silicide may be formed on the polysilicon to reduce itssheet resistance. As the polysilicon is deposited to form the gate 120,the gate 120 forms the lift-up region 122 due to the lift-up region 124in the oxide layer 119. Additional process steps are performed tofabricate the source region (regions 130 and 134), the body contact 136,and the drain region 116.

FIG. 8 depicts an illustrative process to fabricate the illustrativeLDMOS 100 in accordance with embodiments. In step 802, first and secondp-type buried layers are formed in a semiconductor substrate. In step804, a first n-type drift layer is formed above a second n-type seconddrift layer. In step 806 an STI region is formed in the first n-typedrift layer. In step 808 a sacrificial pad oxide layer is grown, and anitride layer is deposited over the sacrificial pad oxide layer.

In step 810 a photoresist layer is deposited, lithographically exposed,baked, and etched to expose an opening to the nitride layer. In step 812the opening to the nitride layer is etched to expose an opening to thesacrificial pad oxide layer. In step 814 a region in the first n-typedrift layer is doped by implanting donors through the opening to thesacrificial pad oxide layer. This region will be part of the JFET regionwhen the p-type body region 128 is formed, and has been referred topreviously as the JFET adjusting implant region.

After the photoresist layer is removed, in step 816 the opening to thesacrificial pad oxide layer is thermally grown. In step 818 the nitridelayer is removed and a body region is formed. In step 820 thesacrificial pad oxide layer is removed (leaving most of the oxide growthof step 816), and a high quality gate oxide is grown. In step 822 a gateis deposited over the oxide, and patterned and etched using aphotoresist layer to form a gate. In the remaining steps, the otherbasic components of the illustrative LDMOS 100 are formed. For example,in step 824 an n-type drain region is formed in the first n-type driftlayer, and an n-type source region is formed in the p-type body region.In step 826 a body contact is formed in the p-type body region.

FIG. 8 summarizes some of the basic steps to fabricate the illustrativeLDMOS 100, but some of the steps are optional for some embodiments. Forexample, some embodiments may not have the JFET adjusting implant region139, and some embodiments may have one p-type buried layer or one n-typedrift layer. Furthermore, the ordering of the steps illustrated in FIG.8 does not necessarily imply a particular ordering of steps in afabrication process.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present disclosure. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

1-6. (canceled)
 7. A method comprising: forming in a semiconductor afirst drift layer having majority carriers of a first type; forming ashallow trench isolation region in the first drift layer; growing a padoxide layer over the semiconductor; depositing a nitride layer over thepad oxide layer; depositing a photoresist layer over the nitride layer;exposing an opening pattern in the photoresist layer; etching an openingin the photoresist layer based on the opening pattern to expose anopening to the nitride layer; etching the opening to the nitride layerto expose an opening to the pad oxide layer; removing the photoresistlayer; growing oxide on the opening to the pad oxide layer; removing thenitride layer; removing the pad oxide layer, leaving at least part ofthe oxide grown on the pad oxide layer; growing a gate oxide layer;forming a gate over the gate oxide layer; forming in the first driftlayer a drain region having majority carriers of the first type; formingin the semiconductor a body region having majority carriers of a secondtype; and forming in the body region a source region having majoritycarriers of the first type.
 8. The method of claim 7, wherein growingthe oxide on the opening to the pad oxide layer includes growing theoxide to a thickness of at least 200 angstroms.
 9. The method of claim7, the method further comprising: implanting dopants through the openingto the pad oxide layer to provide majority carriers of the first type.10-11. (canceled)
 12. The method of claim 7, wherein the semiconductorcomprises silicon and the oxide, pad oxide layer, and gate oxide layereach comprises silicon dioxide, wherein growing the oxide on the openingto the oxide pad layer comprises oxidizing the semiconductor.
 13. Themethod of claim 7, further comprising: forming in the semiconductor asecond drift layer having majority carriers of the first type. 14.(canceled)
 15. The method of claim 13, further comprising: forming inthe semiconductor a first buried layer having majority carriers of thesecond type.
 16. (canceled)
 17. The method of claim 15, furthercomprising: forming in the semiconductor a second buried layer havingmajority carriers of the second type. 18-20. (canceled)
 21. A methodcomprising: forming a doped layer at a surface of a semiconductorsubstrate; filling a trench within the doped layer with a trench oxide,the trench having a corner at which a trench sidewall meets a surface ofthe doped layer; forming an oxide layer over the semiconductorsubstrate, including along the surface of the doped layer, the oxidelayer having a first thickness; forming a gate structure on the oxidelayer over the doped layer and the trench, including over the trenchcorner; and forming between the gate structure and the trench corner araised portion of the oxide layer that has a second thickness greaterthan the first thickness.
 22. The method of claim 21, wherein the firstthickness is about 10 nm, and the second thickness is greater than about20 nm.
 23. The method of claim 21, wherein the second thickness is atleast about 20 nm.
 24. The method of claim 21, wherein forming theraised portion includes thermally oxidizing a portion of the doped layerexposed by a window in a mask formed over the substrate.
 25. The methodof claim 24, further comprising implanting an additional dopant into thedoped layer through the window.
 26. The method of claim 21, furthercomprising forming a doped region within the doped layer and underlyingthe trench corner.
 27. The method of claim 26, further comprisingforming a source region and a drain region having a first conductivitytype at the surface of the substrate, the doped region being locatedbetween the source region and the drain region
 28. A method comprising:forming a dielectric-filled trench at a surface of a semiconductorsubstrate; forming a doped region of the semiconductor substrateadjacent the dielectric-filled trench, wherein the doped region and thedielectric-filled trench share an interface that has a terminus at thesurface of the semiconductor substrate; forming an oxide layer having athickness over the semiconductor substrate, including along the surfaceof the doped region, the oxide layer having a first thickness; andselectively increasing the thickness of the oxide layer over theterminus to a second greater thickness.
 29. The method of claim 28,wherein the first thickness is about 10 nm, and the second thickness isgreater than about 20 nm.
 30. The method of claim 28, whereinselectively increasing the thickness includes thermally oxidizing aportion of the doped region exposed by a window in a mask formed overthe substrate.
 31. The method of claim 28, further comprising forming agate over the doped region and the terminus.
 32. The method of claim 28,wherein the doped region is a portion of a laterally diffused MOSFET(LD-MOSFET), and the doped region is a drift region of the LD-MOSFET.33. The method of claim 28, wherein the doped region is a first dopedregion having a conductivity of a first type, and further comprisingforming a second doped region having the first conductivity typeconnected to the first doped region and underlying the trench.